The Cell Cycle & Differentiation

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  • Created by: ernily
  • Created on: 22-04-15 20:15

The Cell Cycle & DNA Replication

  • The cell cycle consists of a period of cell growth and DNA replication called interphase, and then a period of mitosis.
  • Interphase:
  • DNA helicase breaks the hydrogen bonds between the two polynucleotide strands. The helix unzips to form 2 single strands.
  • Each original single strand acts as a template. Free DNA nucleotides join to the exposed bases.
  • New nucleotides are joined together by DNA polymerase. Hydrogen bonds form between bases on the original and new strand.
  • Each new DNA molecule contains one strand from the original DNA molecule, and one new strand.
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Cell Division - Mitosis

  • Mitosis has five division stages:
  • Interphase:
    • Cell's DNA is unravelled and replicated to double it's genetic content.
    • The organelles are also replicated.
  • Prophase
    • Chromosomes condense. Tiny protein bundles called centrioles start moving to opposite ends of the cell.
    • They form a network of protein fibres called the spindle.
    • The nuclear envelope breaks down and the chromosomes lie free in the cytoplasm.
  • Metaphase:
    • Chromosomes line up along the middle of the cell and become attached to the spindle by their centromere.
  • Anaphase:
    • The centromeres divide, separating each pair of chromatids.
    • The spindles contract, pulling the chromatids to the opposite ends.
  • Telophase:
    • Chromatids reach the opposite poles on the spindle and uncoil, forming chromosomes.
    • The nuclear envelope forms around each group of chromosomes, so there's two nuclei.
    • The cytoplasm divides to form two genetically identical daughter cells.
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Cell Differentiation & Organisation

  • Multicellular organisms are specialised. The structure of each specialised cell is adapted.
  • The process of becoming specialised is called differentiation.
  • Palisade Mesophyll Cells:
    • Contain many chloroplasts, so they can absorb lots of sunlight.
    • The walls are thin, so CO2 can easily enter.
  • Similar cells are organised into tissues, for example:
    • Squamous epithelium tissues.
    • Phloem Tissue.
    • Xylem Tissue.
  • Tissues Are Organised Into Organs - e.g. The Leaf:
    • Lower Epidermis: Contains stomata for gas exchange, it lets air in and out.
    • Spongy Mesophyll: Full of spores to let gas circulate.
    • Palisade Mesophyll: Photosynthesis occurs here.
    • Xylem: Carries water to the leaf.
    • Phloem: Carries sugars away from the leaf.
    • Upper Epidermis: Covered in a waterproof waxy cuticle to reduce water loss.
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